Support for building with GHC-8.4

[alexanderkjeldaas]

I'm getting:

--  While building custom Setup.hs for package packunused-0.1.2 using:
      /home/cibuild/.stack/setup-exe-cache/x86_64-linux/Cabal-simple_mPHDZzAJ_2.2.0.1_ghc-8.4.2 --builddir=.stack-work/dist/x86_64-linux/Cabal-2.2.0.1 build --ghc-options " -ddump-hi -ddump-to-file -fdiagnostics-color=always"
    Process exited with code: ExitFailure 1
    Logs have been written to: /code/.stack-work/logs/packunused-0.1.2.log

    Configuring packunused-0.1.2...
    Preprocessing executable 'packunused' for packunused-0.1.2..
    Building executable 'packunused' for packunused-0.1.2..
    [1 of 2] Compiling Paths_packunused ( .stack-work/dist/x86_64-linux/Cabal-2.2.0.1/build/packunused/autogen/Paths_packunused.hs, .stack-work/dist/x86_64-linux/Cabal-2.2.0.1/build/packunused/packunused-tmp/Paths_packunused.o )
    [2 of 2] Compiling Main             ( packunused.hs, .stack-work/dist/x86_64-linux/Cabal-2.2.0.1/build/packunused/packunused-tmp/Main.o )

    /tmp/stack14128/packunused-0.1.2/packunused.hs:100:23: error:
        Not in scope: data constructor `SimpleUnitId'
        |
    100 |     isInPlacePackage (SimpleUnitId (ComponentId comp)) =
        |                       ^^^^^^^^^^^^

    /tmp/stack14128/packunused-0.1.2/packunused.hs:100:37: error:
        Not in scope: data constructor `ComponentId'
        Perhaps you meant variable `MN.components' (imported from Distribution.ModuleName)
        |
    100 |     isInPlacePackage (SimpleUnitId (ComponentId comp)) =
        |                                     ^^^^^^^^^^^

    /tmp/stack14128/packunused-0.1.2/packunused.hs:121:31: error:
        Ambiguous occurrence `helpHeader'
        It could refer to either `Options.Applicative.helpHeader',
                                 imported from `Options.Applicative' at packunused.hs:27:1-36
                                 (and originally defined in `Options.Applicative.Help.Types')
                              or `Main.helpHeader', defined at packunused.hs:88:1
        |
    121 |                       (header helpHeader <>
        |                               ^^^^^^^^^^

    /tmp/stack14128/packunused-0.1.2/packunused.hs:123:40: error:
        Ambiguous occurrence `helpFooter'
        It could refer to either `Options.Applicative.helpFooter',
                                 imported from `Options.Applicative' at packunused.hs:27:1-36
                                 (and originally defined in `Options.Applicative.Help.Types')
                              or `Main.helpFooter', defined at packunused.hs:51:1
        |
    123 |                        footerDoc (Just helpFooter))
        |                                        ^^^^^^^^^^

[alexanderkjeldaas]

I guess this might be a Cabal update issue..?